Nodular Cast Iron, such as ductile case iron grade GGG40 (DIN), is a common material used to manufacture some inner casings of low pressure steam turbines used in power generation. Such machines may overtime experience water droplet erosion in their inner casings, in particular in regions diagonally opposite the tip of last stage rotating blades where condensed steam is typically centrifuged from the last stage blades to the inner casing at the tip region of the last stage blade so as to impact and erode the inner casing. While the same phenomena may occur at other upstream turbine stages, the problem is greatest at the last stage as this region typically has the highest concentration of condensation.
The erosion reduces the casing wall thickness, which may necessitate repair of the surface. It is generally accepted that materials erosion resistance generally increases with increasing hardness and brittleness. While such materials can be applied by means of laser depositing methods, as welding deposition is generally faster and less expensive, it may be beneficial to use welding deposition method. However, hard and brittle material and are generally difficult to form and are either difficult or impossible to weld.
An alternative to laser deposition involves the fitting of an erosion protection ring which is inserted into a region of the inner casing most effected by the erosion. As the ring material is not limited by the limitations of welding, the ring may be made of ferritic steel having greater resilience to water droplet erosion than nodular cast iron, thus significantly increasing the life of the inner casing. However, while having some advantages, the method may be more expensive than a suitable welding alternative—if developed—and further has the same disadvantage as laser deposition methods of having a longer repair/installation time than welding methods.
A known material suitable for weld repair is work hardening stainless steels. The advantage of this type of material is that, when applied, it may have a hardness of 200 Brindell, making it suitable for welding. Nonetheless, under severe impact and/or compressive abrasion, the hardness of the material may be increased to 500 Brindell. Up until now, it was believed that conditions with a steam turbine would not provide the required cold work hardening of the material, thus leaving the material susceptible to abrasion and erosion.